This is Hungerford, a large female snowy owl. Last summer she was just a hatchling — a gray ball of fuzz in the middle of the Arctic tundra. In the fall, newly equipped with adult plumage, she flew thousands of miles south until she reached the coast of Maryland. And this winter, she became an important part of an unprecedented research project.

'Something Huge Is Going On'

Snowy owls are among the largest birds in North America, but scientists know very little about their behavior. The owls spend most of their days far from humans, hunting rodents and birds in the flat expanses of the Arctic Circle. In the winter, the owls move south, but they don't usually reach the United States. Most years, only a few are spotted in the northernmost states — a rare treat for birders. But this winter was different.

Owls started to appear all over the United States right around Thanksgiving — in Nebraska, in Kentucky — even as far south as Georgia. Dave Brinker, a biologist with the Maryland Department of Natural Resources, was shocked when he saw not one but two snowy owls on a small stretch of Maryland beach.

"Something huge is going on," Brinker told his colleagues. "We won't see something like this for a long time — probably for the rest of our lifetimes."

This rapid population boom — called an "irruption" by ecologists — is the largest the East Coast has seen in 40 or 50 years.

The Trigger (Lemmings!)

So what caused the irruption? This picture tells the story:

It shows a snowy owl's nest that was photographed by biologists last summer in northern Quebec. Those furry lumps are the carcasses of 70 lemmings. Lemmings — small, hamsterlike rodents — are the main source of food for snowy owls in the Arctic. Thanks to the abundance of lemmings last summer, well-fed mother owls laid more eggs, and huge numbers of owlets grew up fat and strong. Come winter, they spread far to the south.

This irruption has provided a once-in-a-lifetime opportunity for owl researchers — the chance to tag and track members of a mysterious species. Brinker says that following the movements of a few snowy owls will give scientists unprecedented information — about their routes through Canada and around the Arctic Circle, about their hunting patterns, and about the human-made hazards they face. That new understanding, in turn, will help the scientists better protect the owls.

The Trap

But they still had to catch the owls. The researchers began by visiting places where snowy owls had been regularly spotted — often flat, open areas like beaches and airports that look a bit like arctic tundra. Once they spotted an owl, they set up a trap nearby, and then ... they waited.

Weidensaul and Brinker both use a trap called a bow net. It looks a bit like a hockey goal — a large rectangular frame that supports a loose netting. The bait is a pigeon. It's outfitted with a sturdy leather jacket to protect it from piercing talons. When an owl swoops in to snatch the pigeon, the net arcs up over it, harmlessly pinning it to the ground.

Project Snowstorm volunteer Steve Huy captured Hungerford on Assateague, an island off Maryland's coast. Huy is an analyst for Marriott International, but he has special training — and a permit — to trap and handle owls.

Huy and Brinker worked together to measure Hungerford's wingspan, tail length and fat stores. They carefully placed a metal band around her leg so she can be identified if she is ever recaptured. They took feather and blood samples that will help them learn more about Hungerford's diet, genetics and exposure to man-made toxins.

Hungerford was still and calm through most of the process, staring around the small cabin that serves as Assateague's research headquarters. But every once in a while, she let out a high-pitched twitter of disapproval and nipped at Huy's hands with her sharp beak.

The Transmitter

Finally, Brinker and Huy fitted Hungerford with a GPS transmitter. It's solar-powered and weighs just 40 grams, less than 3 percent of Hungerford's weight. She wears it like a backpack, with two custom-sewn Teflon straps wrapped snugly around her shoulders so they don't interfere with flight.

"At first they're going to notice it," Brinker says, "but they get used to it pretty quick."

Before releasing Hungerford back into the wild, Huy tried to give her some extra fuel in the form of a dead mouse. Maybe it was the stress of capture or maybe it was too early in the day for her to eat, but she wasn't interested.

Huy released Hungerford on the beach where she was captured, and she glided out into the dunes.

Already, her GPS unit was collecting data: longitude, latitude and altitude, every half-hour. And from Minnesota to Massachusetts, the transmitters of the 15 other owls tagged so far do the same. Every three days, they connect to a cellphone network and send all of their data back to Project Snowstorm.

Hungerford's first few data points show her spending the rest of the day among Assateague's dunes, but after sunset she flew north. Soon, snowy owls all across the United States will join her, starting the long journey back to their arctic breeding grounds. And a few of them, like Hungerford, will carry a high-tech accessory.

This winter has brought an unexpected invasion from the North, the largest migration of snowy owls observed in decades. The Arctic birds usually winter in Southern Canada, but this year they've been spotted as far south as Florida. As NPR's Adam Cole reports, scientists are taking advantage of this unexpected visit by equipping a few of the owls with GPS tracking devices.

ADAM COLE, BYLINE: It's a chilly winter evening on the coast of Maryland, but naturalist Scott Weidensaul and two volunteers are taking a walk on the beach. One of the volunteers cradles a pigeon.

SCOTT WEIDENSAUL: Looks like the lunatics have escaped the asylum here. He's taking his pet pigeon for a walk and I'm walking around with a giant mouse trap over my arm.

COLE: That giant mousetrap actually looks more like a hockey goal, a light frame that supports a loose netting. It's designed to harmlessly capture large birds of prey and the pigeon is the bait. It will sit at the center of the trap wearing a sturdy leather jacket to protect it from piercing talons. A few hundred yards away there's a bright patch of white perched on the crest of a dune, a large female snowy owl.

COLE: He sets up the trap as stealthily as he can and then he waits for the owl to strike. Like most birders, Weidensaul was surprised when snowy owls suddenly started showing up in the United States, mostly in the North and the East, around Thanksgiving.

WEIDENSAUL: There was no preliminary. They're just suddenly, there were snowy owls everywhere. And actually I got a call from Dave.

COLE: That's Dave Brinker, a biologist with the Maryland Department of Natural Resources.

WEIDENSAUL: Who said this obviously more than just the typical every three to five year irruption.

COLE: That's irruption with an I. It means a sudden increase in the abundance of a species. This year's snowy owl irruption wasn't caused by the bouts of freezing weather we've been having or by climate change. It was caused by lemmings. Owls eat lemmings and Dave Brinker says when there are more lemmings, the owls have more babies.

DAVE BRINKER: When they get a big lemming high, they can lay up to 14 eggs and that's what happened this summer.

COLE: Huge numbers of owlets hatched and come winter they spread far to the south.

BRINKER: And that's when Scott and I got on the phone and said, we need to take advantage of this opportunity 'cause we won't see something like this for a long time, probably in the rest of our lifetime.

COLE: So Brinker and Weidensaul launched Project Snowstorm, their goal to tag as many owls as possible with GPS transmitters. By studying the owls' movements - for example, the fact that they hang out at airports, the scientists hope to better understand their behavior and protect them from hazards. In less than two weeks, Project Snowstorm managed to cobble together a national team of volunteers and $20,000 in donations.

Which brings us back to that Maryland beach where Weidensaul has been waiting for more than an hour.

WEIDENSAUL: Here she comes, here she comes, here she comes.

COLE: The big female owl launches herself at the trap, hovers above it for just a moment and then flies back to her perch. After another half hour she flies inland and out of sight.

WEIDENSAUL: After all that. And the next time somebody quotes Dave Brinker to me that anything worth doing is never easy, I'm going to hit them.

COLE: But further down the beach, another birder, Steve Huy, does manage to catch an owl.

STEVE HUY: This is very large. It's a female, pretty good size.

COLE: Like many members of Project Snowstorm, Huy isn't a professional ornithologist.

HUY: I'm an analyst for Marriott International.

COLE: But he is specially trained and permitted to catch owls. Huy and Brinker measure the big female's wingspan, tail length, fat stores and then they take a blood sample. The owl is surprisingly calm, but every so often she voices her disapproval.

HUY: She says, oh, you've got cold fingers.

COLE: And sometimes she shows her disapproval with her beak.

HUY: Ow, ow, ow, ow, ow. Man, that hurts.

COLE: Then comes the GPS transmitter. It's a small light box that fits easily in the palm of your hand and it's equipped with a solar panel and a chip that can hold five years worth of data. The owl wears it like a backpack with two thin Teflon straps wrapping around her wings. Brinker carefully sews the straps by hand so they fit the owl's frame and don't interfere with flight.

BRINKER: At first they're going to notice it, but they get used to it pretty quick.

COLE: As soon as the transmitter is snug, the owl is released on the beach where she was caught. Are there any solemn words spoken on a time like this?

BRINKER: This is a high tech owl so we'll use "Star Trek." Live long and prosper.

COLE: The owl takes off, gliding out into the dunes. Already her GPS unit is collecting data - longitude, latitude and altitude every half hour, and every three days her transmitter and the transmitters of the other 15 owls tagged so far will connect to a cell phone network and send all their data back to Project Snowstorm.

COLE: You can follow all the tagged owls on the Project Snowstorm website. The big female tagged on Maryland's coast has already started her journey back up to the Arctic Circle. At last check-in she was hunting in a Pennsylvania field. Adam Cole, NPR News.